Airborne vehicle nap-of-the-earth (NOE) flight requires precise pilot control to avoid obstacles and elevated terrain. While the pilot normally relies on good visibility to perform the NOE function, requirements exist for NOE flight during periods of less than good visibility (poor weather or nighttime conditions) and (pilotless) automatic guidance. Airborne navigation systems are typically of the dead reckoning (DR) variety, usually based upon doppler radar and compass inputs. Although these navigation schemes offer excellent short term guidance capability, they exhibit long term position error growth, so that an in-flight correction procedure, such as periodic updating of the dead reckoning navigation system, is necessary. For this purpose, terrain correlation techniques, such as TERCOM and SITAN navigation signal processing schemes, have been proposed, as described in my copending patent application Ser. No. 864,843 entitled "MAP-AIDED NAVIGATION SYSTEM EMPLOYING TERCOM-SITAN SIGNAL PROCESSING", filed May 20, 1986, and assigned to the assignee of the present application now U.S. Pat. No. 829,304, issued May 9, 1989. As described therein, by taking advantage of a digital map database, it is possible to provide a computationally efficient navigation system, that improves upon conventional TERCOM and SITAN signal processors, by using elevation and slope data accessed from the digital map database, in conjunction with altimeter inputs, to provide parameter control information for a correlation/modified Kalman filter processor.